1. Field of the Invention
The present invention generally relates to a radio communications system for carrying out packet transmission, and more particularly, to a technique of allocation of radio resources to a downlink packet, in compliance with the service quality of the wireless terminal.
2. Description of Related Art
It is indispensable for a wireless communications system aiming to realize multimedia services to appropriately control packet transmission, taking into account the quality of services (QoS) that differs among applications. In addition, it is expected in the future wireless communications systems that information will be transmitted as packets through the network having wireless links. One technique for supporting a wide variety of service qualities required in a packet wireless communications system is to classify the packets in accordance with the service qualities required by the respective wireless terminals, and to determine the transmission priority for the packets based on the classification.
One known classification method is to set the packet transmission rate to one of the prescribed rates, such as CBR (constant bit rate), VBR (variable bit rate), or ABR (available bit rate) in the ATM (Asynchronous Transfer Mode). Another technique is to use the Diffserv architecture proposed by the IETF (Internet Engineering Task Force). Under the Diffserv architecture, the traffic type is roughly classified into EF (expedited Forwarding), AF (assured forwarding) 1, AF2, AF3, AF4, and BE (best effort). A DSCP (Diffserv code point) value representing the identifier of the QoS class is assigned to each of the traffic types. Based on this classification, a packet is grouped into one of the traffic types in accordance with the service quality required for that packet.
FIG. 1 is a table defining the relation between the traffic types and the transmission priority under the Diffserv architecture. The top priority in data transmission is given to the EF (expedited forwarding) class. The EF class is used for the packet requiring real-time transmission, such as voice. The AF class has the priority next to the EF class, and weighted by four different factors. AF4 class is used for the packet containing video data that requires real-time transmission, such as video conference or streaming. AF1 through AF3 are applied to packets that do not contain data requiring real-time transmission. BE class is at the same priority level as the router that does not support the Diffserv architecture. The BE class is used to the packets containing data that do not require real-time transmission.
An identifier representing the QoS is written in advance in the prescribed field of the header of a packet or a cell. Based on this identifier, a packet transmission control apparatus classifies the packet in accordance with the required service quality, and carries out transmission priority control when forwarding the packet to the destination. For example, since in the Diffserv architecture the top priority is given to the EF class, a packet of the EF traffic type is treated with the top priority.
However, the above-described method may not exhibit its effect efficiently, depending on the environment, if it is applied to radio communications as it is. In other words, even though the packet transmission control apparatus classifies the packets into groups and controls the order of transmission priority based on the required service qualities, a receiver or destination equipment may not receive the exact service quality that the destination equipment requires.
FIG. 2 illustrates an example of the radio communication system. The mobile communications system shown in FIG. 2 includes a base station 504, which functions as a packet transmission control apparatus, and three wireless terminals 501, 502, and 503, which function as receivers or destination equipment. The wireless terminals 501 and 502 are installed with an application corresponding to traffic type AF4, and wireless terminal 503 is installed with an application corresponding to traffic type AF3 in the Diffserv architecture.
The base station 504 has FIFO (First-In First Out) transmission buffers for the respective traffic types. When receiving a packet, the base station 504 stores the packet in the associated transmission buffer corresponding to the traffic type of the addressed terminal one by one in order of arrival. Then, the base station 504 assigns radio resourced to the packet stored in the buffer and transmits it to the destination.
However, under the radio communication environment, the locations (or the circumstances) of the wireless terminals differ from one other, while each location moves frequently as time passes. In addition, the transmission conditions, such as path conditions or interference, often change. Accordingly, the quantity of radio resources required for the packet transmission also fluctuates as time passes.
For example, it is assumed that the radio resource to be allocated is transmission power of the base station 504, and that the transmission power is allocated to AF3 and AF4 classes so that the transmission ratio, that is, the ratio of the number of packets of AF3 to the number of packets of AF4, becomes four to two (4/2).
As illustrated in FIG. 3, the transmission buffer for AF4 class stores packets A-1 through A-3 addressed to wireless terminal 501, which correspond to two AF4-class fields, and packets B-1 through B-4 addressed to wireless terminal 502, which also correspond to two AF4-class fields. The packets are stored in order of arrival of the packets. In this example, AF4 packets are received at the base station 504 in order of A-1, B-1, B-2, A-2, B-3, A-3, B-4. On the other hand, the transmission buffer for AF3 class stores packets C-1 through C-4, which correspond to two AF3-class fields. The arrival order of these AF3 packets is C-1, C-2, C-3, and C-4. The vertical length of the packets shown in FIG. 3 represents the transmission power required to transmit these packets to the destinations.
Based on the transmission ratio and the arrival order, the base station 504 tries to assign transmission power to packets A-1, B-1, B-2, A-2, C-1, and C-2, simultaneously. However, as illustrated in FIG. 4, the required transmission power levels simultaneously assigned to these packets exceeds the maximum transmission power level of the base station 504. A portion of the packets (in this example, packet C-2) cannot receive the necessary power, and consequently, transmission delay occurs, whereby the predetermined transmission ratio cannot be maintained. In other words, the required service quality cannot be satisfied. For this reason, it becomes important for priority transmission control to allocate radio resources in a reliable manner, taking into account changes in transmission paths, interference, and other factors under the radio communications environment.
In addition, since the radio resources, such as time slots, frequency bands, spreading codes, or transmission power, are limited, it is also important to use the finite radio resources efficiently.